SUMMARY

Cyanobacteria are unique prokaryotes, since they in addition to outer and plasma membranes contain the photosynthetic membranes (thylakoids). The plasma membranes of Synechocystis 6803, which can be completely purified by density centrifugation and polymer two-phase partitioning, have been found to be more complex than previously anticipated i.e. they appear to be essential for assembly of the two photosystems. A proteomic approach for the characterization of cyanobacterial plasma membranes using 2-D gel electrophoresis and mass spectrometry analysis revealed totally 57 different membrane proteins out of which 17 are integral membrane spanning proteins. Among the 40 peripheral proteins 20 are located on the periplasmic side of the membrane while 20 are on the cytoplasmic side. Among the proteins identified are subunits of the two photosystems as well as Vipp1 which has been suggested to be involved in vesicular transport between plasma and thylakoid membranes and thus relevant to the possibility that plasma membranes are the initial site for photosystem biogenesis. Four subunits of the Pilus complex responsible for cell motility were also identified, as well as several subunits of the TolC and TonB transport systems. Several periplasmic and ATP-binding proteins of ABC transporter